This patent application is related to co-pending U.S. patent application Ser. No. 08/153,724, filed on even date herewith, entitled "METHOD AND APPARATUS FOR MULTIPLE REPLY REJECTION WHEN DECODING TRANSPONDER REPLY SIGNALS"; and Ser. No. 08/153,722, also filed on even date herewith, entitled "METHOD AND APPARATUS FOR ASSOCIATING TARGET REPLIES WITH TARGET SIGNATURES"; both co-pending applications being commonly owned by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference.
The invention relates generally to receiving and decoding reply signals transmitted from aircraft transponders. More specifically, the invention relates to compensating for phase error introduced into the received and/or processed signals by using calibration signals having a known phase relationship.
Air traffic control and safety are ongoing concerns in commercial and military aviation. Particularly significant concerns are traffic alert and collision avoidance between aircraft either in route between or in the vicinity of landing fields. Ever increasing air traffic demands have resulted in governmental regulations that require commercial carriers to equip planes with active interrogation systems that can determine the presence and threat of nearby aircraft. The particular system mandated by the government depends on the aircraft size. Large commercial aircraft that carry over 30 passengers are being equipped with an active traffic and collision avoidance system (TCAS II) that not only detects and displays nearby aircraft, but also alerts the crew as to impending collisions, and also provides resolution advisories such as audible instructions to the pilot to pull up or down, maintain level or climb rate and so forth. This system, however, is very complex and expensive and therefore has not been mandated for smaller aircraft.
For aircraft that carry up to 30 passengers, governmental regulations require such aircraft be equipped with an active interrogation system (TCAS I) that detects nearby aircraft, determines and displays range, bearing and altitude of such aircraft relative to the interrogating plane, and tracks such aircraft within a prescribed range and also issues an audible alert. Although the operational performance of the TCAS I system appears less complex than TCAS II, numerous problems arise that make a cost effective system difficult to realize.
The Federal Aviation Administration (FAA) specifies that the TCAS I active interrogation systems use air traffic control radar beacon system (ATCRBS) signals. These ATCRBS interrogation signals are high frequency pulse modulated signals at 1030 megahertz. The reply signals are also pulse modulated but at a carrier frequency of 1090 megahertz. In TCAS I, the reply and interrogation signals are transmitted from an interrogation aircraft to other aircraft in the vicinity thereof, and these other aircraft reply to the interrogations via a transponder located on the aircraft.
The interrogation and reply signal waveforms are specified by the FAA. The information contained in the reply signal depends on the type of interrogation (e.g. Mode A, Mode C) and the transponder equipment that the interrogated aircraft has available for responding. For TCAS I, the interrogation mode presently is Mode C, and the Mode C reply signal from the aircraft transponder consists primarily of encoded altitude data. The data is encoded using binary logic states or bits arranged in four octal digit codes (i.e. there are twelve data bits with each octal digit defined by three data bits). The reply signal data bits are transmitted within a pair of framing pulses called bracket pulses that indicate (for purposes of TCAS I) the beginning and end of a reply signal from a particular aircraft responding to an interrogation.
The TCAS I system is specified based on the use of these ATCRBS Mode C reply signal waveforms. Thus, an interrogating aircraft may transmit omnidirectionally an interrogation signal at 1030 MHz, and then will "listen" for Mode C reply signals from all aircraft capable of responding by transmitting the bracket pulses and altitude encoded data bit pulses. Some aircraft are not equipped to reply with altitude data (non-altitude reporting, or NAR) and hence only transmit the bracket pulses. Under TCAS I, aircraft within a range of about 34 nautical miles will reply to a Mode C interrogation.
As part of the reply signal detection and decoding process, a TCAS I compatible system is required to determine angle of arrival of the reply signals from responding aircraft. The angle of arrival information is used to establish a bearing on the responding aircraft relative to the interrogating aircraft. Angle of arrival information is generally determined by phase detection techniques using an antenna array that detects reply signals such that the antenna signals can be combined to produce sum and delta channel signals having a relative phase angle that corresponds to a reply signal angle of arrival.
However, numerous sources of phase error in the system can cause errors in the angle of arrival determinations. For example, antenna cable electrical length differences can cause large phase errors in the antenna signals. These cable induced errors can change over time due to temperature, vibration, aging and so on. Cables can be interchanged, and thus result in mismatches that affect phase determinations. Receiver component variations over time and temperature can also introduce unwanted phase errors. Other sources of phase error include non-linearities in the antenna response characteristics (i.e. electrical angle vs. mechanical angle), as well as non-linearities in the phase comparator used to detect the sum and delta channel phase differences. Phase shifts introduced into the phase comparator inputs due to transmission mismatches can also cause significant phase errors. These are a few of the many possible ways that phase error can be introduced into an antenna/receiver system.
The objective exists, therefore, for a traffic alert and collision avoidance system that utilizes a receiver system capable of detecting phase errors and variations so as to compensate the signal processing to produce output information that is compensated for phase errors and changes. Preferably, the phase compensation should be available throughout normal operation of the system.